1. Field of the Invention
The present invention relates to an optical head having a semiconductor laser for use in an optical information recording and reproducing apparatus for optically recording and reproducing information on and from an optical record medium such as optical card and optical disk.
2. Description of the Related Art
In an optical information recording and reproducing apparatus for optically recording and reproducing information on and from an optical record medium such as optical card and optical disk, speeds of the recording and reproducing are determined by a relative speed of the optical record medium and an optical head. FIGS. 1A and 1B show a known optical head in which a single light beam is made incident upon a single information track on an optical record medium. FIG. 1A is a front view showing the known optical head viewed in a track direction T in which the information tracks extend and FIG. 1B is a side view viewed in a direction perpendicular to the track direction T. In this known optical head, a tracking error is detected by a three beam method and a focusing error is detected by an off-axis method. A diverging laser light beam emitted from a semiconductor laser, i.e. a laser diode 1 is converted into a parallel light beam by a collimator lens 2. Then, the parallel light beam is converted into a substantially circular sectional beam by a shaping prism 60. The laser beam is further constricted into a light beam having a predetermined diameter by an iris 3, and then the laser light beam is made incident upon a diffraction grating 61, where zero order diffracted light beam (main beam) and +1 and -1 order diffracted light beams (sub-beam) are made incident upon an optical record medium 6 by means of an objective lens 5. In this case, the main beam and sub-beams are passed through off-axis regions of the objective lens 5 which are apart from an optical axis of the objective lens 5. Then, the main beam and sub-beams are reflected by the optical record medium 6 are made incident upon a mirror 7 through an opposite off-axis region of the objective lens 5 as clearly shown in FIG. 1B. The light beams reflected by the mirror 7 are then made incident upon a photodetector 9 through an imaging lens 8.
In the known optical head shown in FIGS. 1A and 1B, one main beam spot 62 and two sub-beam spots 63, 64 are formed on the optical record medium 6. The two sub-beam spots 63, 64 are used for a tracking servo, and only the one main beam spot 62 is utilized to record and reproduce the information on and from a single information track on the optical record medium 6. Therefore, this optical head does not provide so high speed for recording and reproducing information with respect to the optical record medium because the recording and reproducing are performed for every information tracks.
To solve such a drawback, there has been proposed another optical head illustrated in FIGS. 2A and 2B, in which the information is recorded or reproduced on and from a plurality of tracks by illuminating these tracks simultaneously. Such an optical head has been described in U.S. patent application No. 07/958,180 filed on Oct. 8, 1992. FIG. 2A is a side view showing this optical head viewed from the track direction T and FIG. 2B is a front view viewed from the direction perpendicular to the track direction T. In this known optical head, the diffraction grating 61 of the known optical head in FIGS. 1A and 1B is replaced by a cylindrical lens 65 which serves to convert the laser beam having a circular cross section into a slit-shaped laser beam. In a plane of the drawing of FIG. 2B in which the cylindrical lens 65 does not have a lens function, the light beam is not refracted by the cylindrical lens and is made incident on the objective lens 5 as a parallel light beam and is projected onto a recording surface of the optical record medium 6 as a fine light spot 66 whose size is determined by a diffraction limit of the objective lens 5. This width of the optical beam spot in this direction is substantially equal to a spot diameter obtained in the known optical head in which a single track is illuminated by a single beam.
In a plane of the drawing of FIG. 2A in which the cylindrical lens 65 has the lens function, the parallel light beam is slightly diverged by the negative power of the cylindrical lens 65 and is made incident upon the objective lens 5. Therefore, the diverged light beam is focused by the objective lens 5 at a point which is apart from the focus point of the objective lens 5, so that a length of the beam spot 66 on the optical record medium 6 becomes longer than a width of the beam spot 66 viewed in the track direction T. In this manner, the light beam spot 66 on the optical record medium 6 is of the slit-shaped one, and a direction of the length of the beam spot 66 is set to be perpendicular to the track direction T, and thus a plurality of information the tracks are simultaneously illuminated.
FIG. 3A is a plan view depicting the slit-shaped laser beam spot 66 formed on an optical card having a number of parallel information tracks, and FIG. 3B is a distribution of an amount of light reflected by the optical card viewed in the direction F--F of the length of the slit-shaped beam. On the optical card there are provided a number of parallel information tracks 11 and track guides 12 between successive tracks 11. In each information track 11, information pits 13 shown by black dots are formed in accordance with the content of the information. The light beam impinging upon the optical card is modulated by the track guides 12 and pits 13 and a distribution of an amount of the reflected light beam is changed as shown in FIG. 3B.
The light beam reflected by the optical card is collected by the objective lens 5, reflected by the mirror 7 and is then made incident upon photodetector 9 by means of the imaging lens 8. Therefore, an image 23 of an optical beam spot is formed on a light receiving surface of the photodetector 9 as shown in FIG. 4. As stated above, the optical beam spot is modulated by the track guides 12 and the pits 13, so that images 24 of the track guides 12 and images 25 of the pits 13 are formed in the optical beam spot image 23. That is to say, a distribution of the image 23 formed on the photodetector 9 becomes similar to that of the light beam reflected by the optical card. In the present optical head shown in FIGS. 2A and 2B, a plurality of pit detecting light receiving elements 14 to 18 corresponding to a plurality of tracks are provided in the photodetector 9 to detect a variation of light amount due to the pit image 25. In this manner, a plurality of the tracks are read simultaneously. Furthermore, the photodetector 9 comprises tracking error detecting light receiving elements 19, 20 and focusing error detecting light receiving elements 21, 22 for receiving the images 24 of the guide tracks 12. By suitably processing output signals generated from these light receiving elements 19, 20 and 21, 22, it is possible to derive tracking error signal and focusing error signal to effect the tracking and focusing servo control such that the optical beam spot image 23 and the track guide image 24 therein come into positions shown in FIG. 4 when tracking error and focusing error are not existent.
However in the optical head shown in FIGS. 2A and 2B, the distribution in an amount of the slit-shaped light beam is not uniform over its whole length. That is to say, the distribution in an amount of the light is varied in the direction parallel with the length of the light beam and an amount of light at a central area is larger than that at peripheral areas as shown in FIG. 3B. Therefore, the level of the output signals produced by the pit detecting light receiving elements 14 to 18 is also varied. This results in that a circuit for converting the output signal into a bivalent signal is liable to be complicated. In addition, a reliability of an information signal read from tracks which situate at edge portions of the optical beam spot.